Image forming methods in image forming apparatuses such as a printer apparatus are roughly classified into an electrophotographic method, thermal transfer method, and ink-jet method. Image forming apparatuses using the electrophotographic method are higher in speed, image quality, and quietness than those using other methods, and have recently become popular. Electrophotographic methods are further categorized into various methods. For example, in addition to a well-known multi-transfer method and intermediate transfer member method, there is proposed a multiple development method of overlapping Y, M, C, and K color images on the surface of a photosensitive body to form a full-color image, transferring the full-color image onto a print sheet at once, and thereby forming the image. There is also proposed an in-line method in which image forming means (process stations) for different colors are aligned and images developed by the process stations are sequentially transferred onto a transfer medium (print sheet) conveyed by a transfer belt. The in-line method can increase the speed, and exhibits high print image quality.
When an image is formed by the in-line method, the density of a printed image varies depending on temperature and humidity conditions under which the printer apparatus is used, and the use frequency of the process stations. To correct variations, the image density is controlled. Image density control will be explained.
Image density control has conventionally adopted a means for forming a density patch image of each color on a photosensitive body, intermediate transfer member, or electrostatic transfer belt (ETB), reading the density patch image by a density sensor, feeding back the result to process forming conditions such as a high-voltage condition and laser power, and adjusting the maximum density and halftone characteristic of each color.
In general, the density sensor irradiates a density patch by a light from a light source, and detects the reflected-light intensity by a light-receiving sensor. The reflected-light intensity signal is A/D-converted, processed by a CPU, and fed back to process forming conditions. The purposes of image density control are to keep the maximum density of each color constant (to be referred to as Dmax control hereinafter) and to keep the halftone characteristic linear to an image signal (to be referred to as Dhalf control hereinafter). Dmax control keeps the color balance between colors constant, and at the same time prevents any scattering and fixing errors of a color-overlapped character caused by an excessive amount of toner.
More specifically, in Dmax control, densities of a plurality of density patches formed by changing image forming conditions are detected by an optical sensor. Conditions under which a desired maximum density is obtained are calculated from the detected results, and the image forming conditions are changed. Each density patch is preferably formed not at a maximum density but at an intermediate density. This is because, if the density of each density patch is close to the maximum density in detecting a so-called solid image for forming a density patch, the change width of an output of the sensor upon a change in toner amount becomes narrow, failing to obtain a satisfactory detection precision.
In Dhalf control, image processing of canceling the γ characteristic and keeping the input/output characteristic linear is performed to prevent any failure in forming a natural image due to a shift of the outputted density to an input image signal caused by a nonlinear input/output characteristic (γ characteristic) unique to electrophotography. More specifically, a plurality of density patches corresponding to different input image signals are detected by an optical sensor, and an input image signal from a host computer is so converted as to obtain a desired density on the basis of the relationship between each input image signal and the density of a corresponding density patch. Dhalf control is generally performed after image forming conditions have been determined by Dmax control.
The density patch formed on the above-mentioned ETB is electrostatically recovered by a process device in a cleaning process. In the cleaning process, a bias having a polarity opposite to the charging polarity of toner is applied to a photosensitive body. Toner is attracted to the photosensitive body in a transfer section, and scraped by a cleaning blade similarly to residual transfer toner.
To perform density control at higher precision, the number of density patches is desirably increased as much as possible. When the number of density patches is increased, however, to form density patches over one or more turns of a photosensitive drum, a problem occurs.
When density patches are formed even in the second and subsequent turns of the photosensitive drum, as shown in FIG. 13, density patch portions formed in the second and subsequent turns of the photosensitive drum are influenced (memory effect) by density patches formed in the first turn of the photosensitive drum. No accurate density is output, resulting in a density control error. To prevent this, the density patch range is generally set within one turn of the photosensitive drum even if the number of density patches is increased. As a measure, each density patch may be downsized to form patches as many as possible within one turn of the photosensitive drum. This measure is, however, undesirable because of the following reasons.    1. Influence of Sweeping Phenomenon
The sweeping phenomenon is that a larger amount of toner is used for development at the trailing end of an electrostatic latent image in comparison with the remaining portion, increasing the density. When the density patch is downsized, the area of the sweeping portion to the density patch increases, and a density higher than an actual one is detected at high possibility.    2. Density Sensor Attachment Position Error
When the density patch is downsized and a position deviated from the center of the density patch is detected due to a density sensor attachment position error, the region suffering the sweeping phenomenon is detected at high possibility.